The Design of Rolling Bearing Mountings Part 5 docx

While the splash oil sufficiently lubricates the crown wheel shaft bearings, which have to accommodate low-er loads, inlets and outlets must be provided for the oil for the pinion shaft

Automotive differentials

Design

Spiral bevel-gear drives – with or without intersecting

axes – are now almost always used for front and rear

axle drives Very high axial loads arise which, with

non-intersecting axes, may be several times the

tangen-tial load at the pinion Due to the limited space and

the elevated torque values, the pinion bearings are very

heavily loaded The pinion bearings should provide for

even meshing of pinion and crown wheel under load;

therefore, the pinion bearing arrangement should be as

rigid as possible The pinion is either an overhung or a

straddled arrangement The overhung arrangement is

usually fitted with two tapered roller bearings adjusted

against one another Compact bearing arrangements

(double-row tapered roller bearings with an unsplit

cup or a cup with a flange) are common

The crown wheel is mounted in common with the

dif-ferential The meshing accuracy of the teeth should

vary as little as possible and mounting should,

there-fore, be provided with sufficient rigidity The rigidity

requirements are easier to meet than with the pinion

since more mounting space is available for this

applica-tion and the axial loads are generally lower

Bearing adjustment

Rigid pinion and crown wheel guidance is achieved by

adjusting the bearings against each other with a

pre-load With grey-cast iron housings, thermal expansion

of the shaft increases the preload in nearly all cases

af-ter operating temperature is reached; the preload must,

however, never be such as to exceed the elastic limit of

the bearing material

The opposite applies to aluminium housings, which

are being used more and more because of their

light-ness So, the preload has to be selected such as to

achieve the required rigidity, but the additional

bear-ing loadbear-ing must not significantly reduce the bearbear-ing

life This is the case if the axial preload does not exceed

about half the external axial force Faapplied

Lubrication

Differentials rely exclusively on oil lubrication Bear-ings and gears are lubricated with the same oil Since

the lubricant is subjected to severe stressing in the

spi-ral gearing, hypoid oils with EP additives are used While the splash oil sufficiently lubricates the crown

wheel shaft bearings, which have to accommodate

low-er loads, inlets and outlets must be provided for the oil

for the pinion shaft particularly for the bearing on the flange side Attention should be paid to the oil flow di-rection which is always from the small end to the large end of the tapered rollers The oil ducts have to be

ar-ranged and dimensioned such as to ensure that oil

cir-culates in every speed range

The pinion shaft is normally sealed by means of radial

shaft seals, in some cases in combination with a flinger

sheet

Bearing dimensioning

Fatigue life analysis of the bearings mounted in

diffe-rentials is based on maximum torque and correspond-ing speed as is the case with automotive gearboxes The percentage times at the individual speeds are based on experience This information is then used to determine

the mean index of dynamic stressing The rolling

bear-ings mounted in cars should have an average fLmvalue

of 1 1.3

Wear of these bearings should be minimal since

diffe-rential drives require a high guiding accuracy and as quiet running as possible With today's bearing

di-mensioning the service life of differential bearings is ei-ther terminated by fatigue or wear.

A detailed calculation of the attainable life is usually

not necessary as these bearings have proved their worth sufficiently in the automotive sector Bearing dimen-sioning based on a comparison calculation with the

index of dynamic stressing fLis sufficient

34 Final drive of a passenger car

Operating data

Maximum engine torque 160 N m at 3,000 min–1

Bearing selection

Pinion shaft

The pinion shaft is fitted with FAG inch-dimensioned

tapered roller bearings mounted in O arrangement.

Dimensions: 34.925 x 72.233 x 25.4 mm (dynamic

load rating C = 65.5 kN) and 30.163 x 68.263 x

22.225 mm (C = 53 kN)

The pinion is accurately positioned relative to the

crown wheel by means of shims inserted between

housing shoulder and bearing cup The cones are

cir-cumferentially loaded But only the cone of the larger

bearing can be press-fitted The cone of the smaller

bearing is slide-fitted because the bearings are adjusted

through this ring

Crown wheel

Crown wheel and differential are mounted on the

same shaft Fitted are two FAG inch-dimensioned

tapered roller bearings of 38.1 x 68.288 x 20 mm;

C = 39 kN

Both bearing and gear mesh adjustment are achieved by

means of shims

Machining tolerances

Pinion shaft: m6 (larger-size bearing)

h6 (smaller-size bearing) housing P7

Crown wheel: hollow shaft to r6

housing to H6

To allow the pinion to be adjusted to a certain torque

and to avoid expensive fitting work (for instance ma-chining of a solid spacer), a thin-walled preformed sleeve is provided between the bearing cones The sleeve is somewhat longer than the maximum distance between the two bearing cones Depending on the width tolerance values of the bearings there will be some elastic deformation of the sleeve (a few microns

at most)

34: Final drive of a passenger car

35–39 Automotive wheels

Differences exist between driven and non-driven

wheels for automobiles; the bearings can be either

steerable or non-steerable Basically, all wheels must be

guided as accurately and clearance-free as possible for

driving control reasons This is in most cases achieved

by using angular contact ball bearings or tapered roller

bearings which are adjusted against each other.

Front wheels

Where steered, non-driven front wheels are concerned,

the axle or shaft journal are relieved of torque

trans-mission and can, consequently, be given relatively

small dimensions The tendency towards compact

wheel bearing units is encouraged by the wish for the

smallest roll radius possible as well as the pressure to

reduce weight and to simplify series mounting

Double-row angular contact ball bearings are almost

always selected where the ratio of the mounting space

for the wheel bearings axial width to the radial cross

section height is less than 2.5 The following

advan-tages can then be felt:

– little space is required in the axial direction, a large

spread and, therefore, a high moment load carrying

capacity due to a large contact angle,

– total weight of the bearings is low,

– suitable for integration in bearing units,

– flanges can be more easily integrated – particularly

at the inner ring – than with tapered roller bearings

Rear wheels

With non-steered rear wheels, the radial mounting

space is generally limited not only in the case of

con-ventional drum brakes but also in vehicles with disc

brakes since an extra drum brake is usually mounted at

the rear wheels as a parking brake The actuation

mechanism is inside the drum near the axle and limits,

as a result, the maximum outside diameter of the hub

In comparison, the axial mounting space is normally

not as restricted so the wheel bearings do not have to

be particularly short

Today's standard bearing arrangement for such wheels,

therefore, consists of two relatively small single radial

tapered roller bearings which are mounted at a larger

distance The bearings have small contact angles so that

the highest load rating possible is reached in a small

mounting space The necessary spread to accommodate

tilting forces is achieved with the large bearing

dis-tance

With the wide range of standard tapered roller

bear-ings, this simple bearing arrangement, which is

inex-pensive where solely the bearing costs are concerned,

offers diverse variations for all vehicle types and sizes

There are, however, also some disadvantages particu-larly with large series:

– Numerous single parts must be purchased, stored and mounted

– The bearings have to be greased and sealed during

mounting

– The bearing system must be adjusted and the

adjust-ing elements secured in the correct position Therefore, for rear wheels there is also a tendency to use double-row angular contact ball bearings which do

not have to be adjusted when mounting and which can

easily be integrated in bearing units

Machining tolerances

The outer rings or cups of non-driven wheel bearings

(hub bearings) are subjected to circumferential load (interference fit ) whereas the inner rings or cones ac-commodate point load (loose, sliding or wringing fit);

this facilitates mounting and bearing adjustment The the inner rings or cones of driven wheel bearings

are circumferentially loaded, and the outer rings or cups are point-loaded; this has to be taken into account

when selecting the machining tolerances

Non-driven front or rear wheels with two angular con-tact ball bearings or two tapered roller bearings: inner bearing: shaft to k6 (h6)

hub to N6, N7 (P7 for light-metal hubs) outer bearing: axle journal to g6 j6 hub to N6, N7 (P7 for light-metal hubs) Driven front or rear wheels with double-row angular contact ball bearings (bearing unit):

shaft to j6 k6 hub to N6, N7 (P7 for light-metal hubs)

Bearing dimensioning

For the fatigue life calculation of wheel bearings, the

static wheel load, the dynamic tyre radius rdynand its coefficient of adhesion, as well as the speeds of the ve-hicle in the operating conditions to be expected, are taken into account The loads on the individual bear-ings or – for double-row bearbear-ings on the individual

rolling element rows – are determined with the forces

and moments calculated The calculation results can only be taken as reference values Normally the ideal fL values for passenger cars are approximately 1.5 and for commercial vehicles approximately 2.0

Lubrication, sealing

Wheel bearings are almost exclusively lubricated with

grease Bearings which have no integrated seals are

nor-mally sealed with spring-preloaded shaft seals with

spe-cial dust lips Sealed bearings such as the double-row

angular contact ball bearings with for-life lubrication,

which are widespread in passenger cars, normally have

a combination of dust shield and seal Experience has

shown that these seals are satisfactory if the design

pro-vides an additional gap-type seal Collecting grooves and baffles are also required to protect the bearings against dust and splash water

Operating data

Wheel load 4,600 N; tyre size 175/70 R14;

rdyn= 295 mm; maximum speed 180 km/h

Bearing selection

The bearing arrangement is made up of a sealed

dou-ble-row FAG angular contact ball bearing

The bearing is greased for life with FAG rolling bear-ing grease.

The bearing arrangement of a driven and non-steered rear wheel of a rear drive passenger car may also be de-signed like this

35: Passenger-car front wheel

Driven and non-steered rear wheel

Operating data

Wheel load 4,800 N; tyre size 195/65 VR15;

rdyn= 315 mm; maximum speed 220 km/h

Bearing selection

The wheel bearing arrangement consists of a

double-row FAG angular contact ball bearing which is greased

for life.

Seals and flinger rings provided on both sides protect

the bearing from contamination

Machining tolerances

The inner rings and the outer ring of the bearing are

tightly fitted.

36: Passenger-car rear wheel

37 Driven and non-steered rear wheel of a rear drive truck

The rear wheel hubs of heavy trucks often feature a

planetary gear This type of drive provides a relatively

high gear ratio in a limited space As the high driving

torque is generated directly at the wheel, small

diffe-rential gears and light drive shafts are possible

Operating data

Wheel load 100 kN; tyre size 13.00-20;

rdyn= 569 mm; permissible maximum speed 80 km/h

Bearing selection

Wheel bearings

Tapered roller bearings FAG 32019XA (T4CC095

ac-cording to DIN ISO 355) and FAG 33021 (T2DE105

according to DIN ISO 355) Since these bearings have

a particularly low section height they require only a

small radial mounting space thus allowing light-weight

constructions The relatively large bearing width and

long rollers result in a high load carrying capacity

The bearings are adjusted against each other in O

arrangement (large spread).

Planetary gears The outer planet drive increases the driving torque in a minumum space The planet gear bearing arrangement

is of the full-complement type, i.e it features two rows

of needle rollers Axial guidance is provided by thrust washers

Machining tolerances

Direct bearing arrangement with needle rollers: shaft to h5; housing to G6 Tapered roller bearing: shaft to j6; housing to N7

Lubrication

Common oil lubrication for planet drive and wheel

bearings An oiltight, welded housing protects gear and bearings against contamination

37: Rear wheel of a truck

38 Steering king pin of a truck

A variety of steering king pin mounting arrangements

are possible The bearing arrangement with two

adjust-ed taperadjust-ed roller bearings for accommodating the axial

loads is generally used in driven truck front wheels In

other cases the axial loads are accommodated by thrust

ball bearings or tapered roller thrust bearings Since

the radial mounting space for king pin bearing

mount-ing arrangements is usually very limited the radial

loads (steering and guiding forces) are accommodated

by a plain bearing made of bronze and drawn cup

needle roller bearings which provide for easy steering

Mounting with a tapered roller thrust bearing

The shock loads on the steering king pin are very high

Therefore, the thrust bearing must have a high load

carrying capacity and be mounted with zero clearance

or preload As the king pin performs only slight

slew-ing motions no cage is required so that the number of

rolling elements and, consequently, the load carrying

capacity can be increased

The example features a full-complement tapered roller

thrust bearing as the thrust bearing It has a profiled

shaft-washer raceway and a flat housing-washer race-way The sealed bearing is held together by a pressed steel cap, which simplifies mounting

The bearing is filled with special grease; it can be relu-bricated if necessary Openings in the sealing lip and

the elasticity of the sealing material ensure the escape

of the spent grease.

The clearance between the knuckle and the cross member is compensated for by shims In this way, the thrust bearing can have zero clearance at best, which means higher shock-type loads Experience has shown that this can be taken into account by means of an im-pact factor of fz= 5 6, in the case of adjusted tapered roller bearings with an impact factor of fz= 3 5 The shaft washer of tapered roller thrust bearings is

located by a relatively loose fit on the steering kin pin

(g6); the housing washer has no radial guidance

38: Steering king pin of a truck

39 Shock absorbing strut for the front axle of a car

Front axles are being equipped more and more

fre-quently with McPherson shock absorbing struts

When driving, the coil spring and the damping unit of

the McPherson strut cause movements relative to the

body which are due to spring deflection and the degree

of lock For comfort reasons and for easy handling,

these slewing motions are supported either by rolling

bearings or rubber elements Deep groove ball bearings

best meet all requirements

Bearing selection

Requirements

– Accommodation of weights and high shock loads

– Maintenance-free design

Variants – Damping unit and spring coil rotate together – single path solution (fig a) The spring coil loads and the pulsating loads from the piston rod act on the strut bearing

Possible bearing designs: Deep groove ball bearings

loaded axially (with cage or full-complement

vari-ants with a fracture-split outer ring) or thrust ball bearings

– Movements of the shock absorber's piston rod and

of coil spring are independent of each other – dual path solution (fig b)

Direct connection of shock absorber's piston rod to the body via a rubber element; coil spring supported

by a special thrust ball bearing or angular contact ball bearing (spring seat bearing)

Both variants meet all requirements concerning

seal-ing, for-life lubrication and economic efficiency.

39: Shock absorbing strut for the front axle of a car; a: single path solution; b: dual path solution

40 Water pump for passenger car and truck engines

The water pump provides for circulation of the

cool-ing water in the engine Smaller and lighter pump

de-signs are possible with ready-to-mount bearing units

Bearing selection

The water pump bearing unit consists of the shaft and

a common outer ring with raceways for

rolling-element-and-cage assemblies The example features one

ball-and-cage assembly and one roller-ball-and-cage assembly

each mounted in a locating-floating bearing

arrange-ment The roller-cage assembly is designed as the

float-ing bearfloat-ing at the side that is most heavily loaded by

the belt pull The ball-cage assembly is the locating

bearing: in addition to the radial loads it also

accom-modates the thrust of the pump impeller

Machining tolerance, bearing clearance

The outer ring is mounted into the housing with an

R7 interference fit The bearing clearance of the unit is selected to allow for a small operating clearance.

Lubrication, sealing

For-life lubrication with a special rolling bearing

grease Lip seals in the outer ring are provided on both

sides against grease escape A spring loaded axial face

seal is fitted at the impeller end Unavoidable water

leakage is drained to the outside through the outlet bore

40: Water pump bearing unit for a truck engine

41 Belt tensioner for passenger car engines

The cam shafts of many four-cycle engines are driven

with toothed belts from the crankshaft

The belt tension necessary for quiet running is

provid-ed by an FAG bearing unit This tensioning pulley

unit consists of a journal with integral raceways, a

ball-cage assembly and an outer ring with the plastic

injec-tion-moulded tensioning pulley

The screw bore for fastening the tensioning pulley to the engine housing is eccentrically located so that the belt tension can be applied by rotating the journal

The bearing unit is sealed on both sides and packed with grease for life Speed is approximately

7,000 min–1

41: Belt tensioner for passenger car engines